This invention relates to the twisting of electrical and optical cables and lines and more particularly to the layerwise twisting of corresponding twisting elements with special consideration of the SZ twisting technique.
The development in the field of SZ twisting technology for electric cables and lines which started at the beginning of the sixties has led to SZ twisting methods and SZ twisting machines, with which two to five twisting elements, for instance, conductors for sheathed cables, conductors for pairs, triplets or spiral quads of communication cables, can be twisted together without problem. It is characteristic for the majority of the SZ twisting methods developed, namely those which work with a concentrated or not concentrated accumulator, that the final twisting of the material to be twisted is the result of a superposition of at least two twisting operations of opposite direction, which take place at the entrance and the exit of the SZ twisting device. This applies, for instance, to the twisting with a rotating length accumulator of alternating direction of rotation, to the twisting with a revolving twisting device and changing accumulator content, to the twisting by means of a twisting head arranged at the end of a torsioning section (U.S. Pat. No. 3,593,509) as well as to the twisting with two twisting heads which are arranged at the beginning and the end of a torsioning section and revolve with constant direction of rotation.
The SZ twisting machines nowadays available on the market operate, as a rule, with the material to be twisted stretched taut during the twisting, the material to be twisted being tensionally gripped from the outside in the circumferential direction by means of rotating twisting heads. In the region of the twisting section proper, the material to be twisted is conducted through open air or by means of special guide elements. The rotary motion of the twisting heads changes at distances which are matched to the time interval between the first twisting when entering the twisting section and the second twisting of this length when leaving the twisting section. It is customary to change the rotary motion in steps between two values. The period of these stepwise changes is as a rule twice the mentioned time interval, but can also be an uneven part thereof (German Auslegschrift DE-AS 15 15 730, claim 1).
Since the beginnings of SZ twisting, methods and apparatus have also been considered by which twisting elements can be SZ twisted in layers. Here, a twisting plate revolving with alternating direction of rotation is essentially provided as the twisting means proper (German Pat. No. 631 929, claim 1). It is, however, a particular difficulty in the development of suitable methods and machine designs for this kind of layerwise twisting, that the twisting should lead to spacings of the reversal points of the twist direction as large as possible in the twisted material produced.
For multi-layer twisting of twisting elements of electric cables in one operation, twisting machines are otherwise known which operate conventionally, i.e., with constant rotary motion and constant pull-off velocity and in which the stationary twisting elements are fed to several staggered twisting closers. The twisting is accomplished by a constant rotary motion of the take-up device and of other torsioning devices associated with each twisting closer, and a variation of the length of lay between adjacent twisted layers is achieved by moving one or more twisting closers (German Auslegeschrift DE-AS 14 90 355, claim 1). It is also known to effect the length of the lay variation by means of revolving intermediate accumulators which change their rotary motion at intervals (German Offenlegungschrift DE-OS 18 01 063, claim 1).